Organic EL has recently been attracting attention for application to light-emitting devices and display devices (see Non-patent Document 1 and Non-patent Document 2). Products to which an organic EL light-emitting phenomenon is applied are being put to practical use (for example, see Patent Document 1). There are generally two excited states a light-emitting material can be in, which are a singlet excited state and a triplet excited state, and the former is considered to be probabilistically about one-third of the latter.
The ground state of a light-emitting material is a singlet state, and for ordinary organic molecules, the transition from the singlet excited state to the ground state is possible, but the transition from the triplet excited state to the ground state is forbidden. In other words, although light emission can be obtained through the transition from the singlet excited state to the ground state, in many cases, the direct transition from the triplet excited state to the ground state does not occur, and the transition from the triplet excited state to the ground state is made via various levels. In that process, energy is released not as light but as heat. Such a phenomenon is called thermal deactivation.
As described above, the probability of transition into the singlet excited state is one-third of that of transition into the triplet excited state. Therefore, much of energy given to the light-emitting material is lost as heat via the triplet excited state, which results in poor efficiency. In addition, a large amount of heat is generated, which causes deterioration of an organic material.
In order to solve such problems, a technique has been developed for increasing emission efficiency by adding an organic material containing a heavy metal (particularly, iridium) to a light-emitting material at about 5% (see Non-patent Document 1). In such a technique, a main constituent is called a host, and an organic material added is called guest (or dopant).
In this technique, a host molecule in the triplet excited state returns to the ground state by transferring the state to a guest molecule, and conversely the guest molecule receives the excited state from the host molecule and is brought into the triplet excited state. The guest molecule can return to the ground state from the triplet excited state through light emission owing to spin-orbit interaction.